Progesterone inhibition of estrogen-induced maternal behavior in hysterectomized-ovariectomized virgin rats

Hysterectomized-ovariectomized virgin rats were tested for maternal behavior following treatment with 100 μg/kg EB immediately at surgery and either oil, 0.5 or 5.0 mg progesterone either 0, 24 or 44 hr following surgery. Stimulus pups were presented 48 hr postoperatively which is counted as Day 0 of testing. EB + oil-treated females displayed short-latency maternal behavior beginning on Day 0. The injection of 5.0 mg progesterone at 0, 24, or 44 hr significantly inhibited the onset of maternal care while the effect of the lower dose of progesterone depended upon the timing of its administration in relation to that of EB. At a dose of 0.5 mg, progesterone given 24 hr following EB, inhibited the appearance of maternal behavior but had no effect given at 44 hr, and resulted in only a partial delay when given at the same time as the EB. Possible mechanisms by which progesterone interfered with the display of maternal behavior were discussed.